What Type of Hair Keeps You Cooler, Curly or Straight?

As summer approaches and temperatures rise, you’ve probably thought about different ways to keep cool: wearing the right clothes, drinking plenty of fluids, and limiting time outdoors during the hottest hours.

But have you ever thought about how your hair might protect you from dehydration and overheating?

New research published on June 6 in Proceedings of the National Academy of Sciences evaluated different hair types and found that curly hair provided the most protection from the sun’s heat.

“Humans evolved in equatorial Africa, where the sun is overhead for much of the day, year in and year out,” said Nina Jablonski, PhD, the Evan Pugh University Professor of Anthropology at Penn State in University Park, Pennsylvania, in a press release.

Investigators wanted to understand how the heat generated by hours upon hours of intense sunshine might have affected the evolution of hair. “We found that tightly curled hair allowed humans to stay cool and actually conserve water,” said Dr. Jablonski.

Curls Have Air Pockets to Help With Cooling

To simulate the extreme elements that our ancestors faced, researchers used a thermal manikin — a human-shaped model that employs electric power to simulate body heat and allows scientists to study heat transfer between human skin and the environment — and human-hair wigs of diverse textures.

Scientists programmed the manikin to maintain a constant surface temperature of 95 degrees Fahrenheit, similar to the average surface temperature of human skin, and set it in a climate-controlled wind tunnel.

After taking baseline measurements of body heat loss by monitoring the amount of electricity required by the manikin to maintain a constant temperature, researchers shined lamps on the manikin’s head to mimic solar radiation under four scalp and hair conditions — totally bald, straight, moderately curled, and tightly curled.

The scientists calculated heat loss in a high-heat, high-humidity scenario such as equatorial Africa on all the types of hair. They found that all hair reduced solar radiation to the scalp, but tightly curled hair provided the best protection from the sun’s radiative heat while minimizing the need to sweat to stay cool.

“We suspect that the structure of human curls allows for a lot of air pockets between hair strands. In other words, you can have a lot of depth (from the scalp to the top of the hair) without having a lot of density, which would keep heat in as well as keeping heat out,” says a coauthor of the study, Tina Lasisi, PhD, who conducted it as part of her Penn State doctoral dissertation.

Even Moderately Curled Hair Can Be Cooling

Does hair have to be really curly to offer cooling advantages? According to the experiments, moderately curled hair did offer some benefits, but the exact level of curl that is required to pass some threshold of heat-related benefit still needs to be determined, says Dr. Lasisi.

“Other factors like length and style also require further study, but all our wigs had hair fibers that were eight-inches long and worn loose without any particular styling other than brushing the hair,” she says.

While it is extremely unlikely that differences in hair texture affect survival in modern populations, some of the findings continue to have relevance today, says Lasisi. “The findings highlight the importance of some form of barrier (hair or otherwise) to reduce overheating in the sun,” she says.

Earlier Studies Suggest Our Brains Don’t Work as Well When We Get Too Hot

A small study published in May 2020 in Scientific Reports found that too much sun on the head impacted people’s ability to perform functions that require thinking and coordination. According to the authors, heat affects thinking by undermining the way the nervous system works.

Athletes with tight curls who flatten or straighten their hair may want to consider these results, as they may be decreasing their ability to stay cool by losing heat through their scalp, says Lasisi.

Hot, Humid Weather Poses Numerous Health Risks

Exercising in hot weather, particularly in humid conditions, can increase core body temperature, causing even seasoned athletes to exercise cautiously, says Michael Fredericson, MD, a professor of orthopedic surgery and a sports physician at Stanford Health in Palo Alto, California, who was not involved in this study.

“Your body cools by sweating, but cooling down is more challenging in humid weather because perspiration doesn’t evaporate as quickly, putting you at risk of heat exhaustion or heatstroke,” says Dr. Fredericson.

Heat exhaustion and heatstroke aren’t the same thing, he says. “Heat exhaustion is a milder form of heat-related illness that can develop after several days of exposure to high temperatures and more immediately after inadequate or unbalanced replacement of fluid.” It can usually be remedied by drinking, resting, and finding a place to cool down, according to the Cleveland Clinic.

Heatstroke is a medical emergency in which the body’s cooling system has been overwhelmed by heat and dehydration, and the body temperature may rise to 103 degrees F or higher, says Fredericson. “If a person does not get treatment right away, heatstroke can permanently damage the body’s organs, including the brain,” he says.